The present invention relates to a process for the separation of rhodium from rhodium containing solutions including organic solutions, organic-aqueous mixtures, and aqueous solutions containing organic complexing compounds, by precipitation at elevated temperatures.
Aqueous and non-aqueous rhodium containing solutions are produced in many sectors of industry and the rhodium has to be recovered from these solutions while accompanying substances, such as salts or organic compounds of low volatility, are separated out to the greatest possible extent. Such solutions are especially residues from homogenous rhodium catalysis reactions, spent solutions from electroplating, or mother liquors from the production of organorhodium compounds. Recovery of the rhodium from these waste solutions as quantitatively as possible often plays a large part in the viability of the processes carried out with the rhodium. Problems with the quantitative recovery of rhodium from waste solutions always arise when the rhodium is in an organic or organic-aqueous solution or is held in aqueous solution by strong complexing agents.
The best-known process for the recovery of rhodium from recycling materials or from primary production concentrates is separation by a wet chemical method. This is done by converting the material to an aqueous solution acidified with hydrochloric acid, from which the rhodium is separated out as ammonium hexachlororhodate by precipitation, optionally after the other noble metals have been separated out. The rhodium residues remaining in solution after the ammonium precipitation are separated out by reduction or cementation. This process has the disadvantage that all the material must be converted to an aqueous solution acidified with hydrochloric acid, and that the separation of the rhodium is not quantitative, particularly when comparatively large proportions of base metal or semi-metal salts are present. When other reducible metals are simultaneously present, selective separation of the rhodium residues by cementation is impossible. If the aqueous solution acidified with hydrochloric acid contains other strongly complexing components, neither ammonium precipitation nor cementation can be carried out quantitatively. Organic rhodium solutions or organic-aqueous solutions cannot be recovered by this process.
If the rhodium solutions contain impurities such as base metal salts, neutral salts, complexing agents, or high-boiling organic compounds, a useful method of recovery can be to subject the waste solutions containing noble metal to a metallurgical procedure. However, these processes are generally expensive and cannot recover the rhodium quantitatively. Also, there is only a limited possibility of processing comparatively large quantities of combustible organic compounds via metallurgical processes.
Another possible way of treating combustible waste containing rhodium is first to burn off the organic matrix and then to recover the rhodium from the remaining ash. The disadvantages of this process are atmospheric pollution, the residues of undesirable elements in the ash, and the rhodium losses on combustion.
German patent No. 32 23 501 describes a process in which the noble metals are precipitated from aqueous and non-aqueous solutions by reaction with tellurium or reducible tellurium compounds and recovered from the precipitate. This process has the disadvantage that it is not generally applicable to all rhodium-containing materials, that tellurium occasionally passes into the solvent in relatively large quantities which interfere with further processing, and that, in the reaction, readily volatile and strong-smelling tellurium compounds can be formed which make handling more difficult, degrade the environment, and may be toxic. The tellurium/rhodium concentrate obtained in the treatment of rhodium-containing organic materials by this process must be freed from the adhering organic materials by combustion before the recovery of the rhodium. The tellurium dioxide thereby formed is volatile and not only represents tellurium losses but also gives rise to problems with the waste-gas treatment.